Network nodes often require strict or wide-sense nonblocking N.times.N cross connects. A nonblocking cross connect can connect any of N ports on one side of the cross connect to any of N ports on the other side of the cross connect in any of the N! possible connection setups. A strict or wide-sense nonblocking cross connect can change a pair of connections without interrupting any of the other connections. (See R. Ramaswami et al., "Optical Networks: A Practical Perspective," Morgan Kaufmann Publishers, San Francisco, 1998.) Furthermore, performing the switching optically allows for very large connection bandwidths.
One approach to making an optical cross connect is to use bulk optics with moving parts. Strict-sense nonblocking micro and macro-mechanical space switches with N as large as 8 and 72, respectively, have been reported. (See, e.g., L. Y. Lin et al., "High-Density Connection-Symmetric Free-Space Micromachined Polygon Optical Crossconnects With Low Loss for WDM Networks," Optical Fiber Comm. Conf., paper PD24-1, 1998; and the Astarte company web site at http://www.starswitch.com/7250spec.htm.)
Another approach is to use planar lightwave circuits, which can be completely solid-state. Strict-sense nonblocking space switches in AlGaAs, InGaAsP, LiNbO.sub.3, and silica with N as large as 8, 4, 8 (16 using multiple chips), and 16 (actually only wide-sense nonblocking), respectively, have been demonstrated. (See, e.g., T. Goh et al., "Low-loss and high-extinction-ratio silica-based strictly nonblocking 16.times.16 thermooptic matrix switch," IEEE Photon. Technol. Lett., vol. 10, pp. 810-812, 1998.)
The planar approaches have used either arrangements of 1.times.2 or 2.times.2 switches or a broadcast-and-select architecture. (See M. Gustavsson et al., "Monolithically integrated 4.times.4 InGaAsP/InP laser amplifier gate switch arrays," Electron. Lett., vol. 28, pp. 2223-2225, 1992.) Although such arrangements can have very low crosstalk, they have several drawbacks in terms of scaleability. First, in such arrangements, the number of switches and/or gates is .gtoreq.N.sup.2, consuming nearly an entire wafer for N=16. (See, e.g., Goh et al.) Second, the electrical-lead-number and settings number is .gtoreq.N.sup.2. Third, there are often many waveguide crossings.